Starfish belong to the class
A. Pisces
B. Cephalopoda
C. Asteroidea
D. Ophiuroidea

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Hint:Starfish - they are marine invertebrates, belonging to the phylum Echinodermata. Its scientific name is Asteroidea derived from the Greek word "aster" (a star) and eidos (appearance) by owning five arms. This fish looks like a star.

Complete answer:Basic characteristic features of starfish: They are radially symmetrical, marine and benthic animals. Their bodies are flat and crawl on the sea bottom. It has two sides upper and lower, where we name it according to the presence of mouth or anus, so the mouth on the lower-side is known as the oral side and the upper side has anus called the abdominal side. The aboral side has two openings - anus and madreporite. The madreporite is the opening of the vascular system. Echinoderms have spiny skin because of the presence of an endoskeleton, which is made up of calcareous plates, and also have pedicellariae with movable jaws, which helps in the removal of debris.
Now, if we talk about the water vascular system. For understanding, we have to draw a section of starfish which has been cut horizontally.

                                                  Fig: water vascular system of the starfish.
Here, the starfish has five arms, and the central part is known as the ring canal. There is a small tube with an opening called the madreporite, which opens into a tube and this tube leads into the other tubes which are going into each arm called a radiating or radial tube from the central canal. so, from Madreporite the water enters into the central tube from where the water passes through each arm. Now here we also see a small tube from madreporite to the central canal known as a stone canal. Each radial canal has paired tubes on either side and each tube has a bulb-like structure called tube foot. This tube foot helps in locomotion. Two arms between which the madreporite is present are known as bivium and the remaining three arms together form trivium. The vascular system is also known as the ambulatory system.

Hence, the correct option is option-C.

Note: Economic importance of starfish is that they are dried and added into the soil as a calcium source because of their spiny skin. This spiny skin is because of calcareous plates which are present under the skin. Calcareous means made up of calcium salts and they are edible too.